Method and apparatus for control of a supersonic motor

ABSTRACT

A method for control for a supersonic motor able to control torque independently and an apparatus for the same. In the invention, the mechanical inductance (L 0 ) and mechanical capacitance (C 0 ) are set so as to cancel each other out by series resonance so as to enable operation of the supersonic motor at the mechanical resonance point of its vibrator, (b) a parallel resonance tank circuit (23) is configured using the inductance (L d ) component provided at the outside of the supersonic motor so as to be cancelled out by an electrostatic capacitance (C d ) and use is made of a circuit comprised of a mechanical arm resistance of the supersonic motor and a torque-mirror circuit, and (c) a reference voltage corresponding to the reference torque is supplied to the piezoelectric actuators (13, 14) of the supersonic motor to control the torque of the supersonic motor. The voltage supplied to the piezoelectric actuators (13, 14) driving the vibrator (11) is proportional to the torque. By adding a speed control loop to the outside of the torque-mirror circuit, it is also possible to control the speed of the supersonic motor. Further, by adding a position control loop to the outside of the speed control loop, it is also possible to control the position of the supersonic motor.

TECHNICAL FIELD

The present invention relates to a method and apparatus for the controlof a supersonic motor, more particularly relates to a method andapparatus for the control of a supersonic motor which enablesindependent control of the torque.

BACKGROUND ART

It has been about 10 years since supersonic motors were first reported.

A supersonic motor has superior high torque characteristics. Practicaluse of supersonic motors is possible in various fields using thesecharacteristics.

Up until now, however, no method has been established for accuratelycontrolling the torque produced in the supersonic motor. That is, upuntil now, no one has proposed a method for independent control of thetorque in a supersonic motor. As a result, supersonic motors have notbeen commercially used in applications for control of position and speedlike those of electromagnetic type servo motors, for example, for aspart of a robot arm for increasing or decreasing the speed of movement.That is, the supersonic motors up until now have not, unlikeelectromagnetic type servo motors, allowed the formation of currentloops and therefore have not been able to be directly connected to andoperated in cooperation with the general commercially available robotcontrollers.

Details will be provided below.

A supersonic motor is usually driven near the mechanical resonance pointso as to reduce loss, but right from when supersonic motors were firstreported, attempts have been made to control supersonic motors bychanging the sensitivity of the motors by deliberately shifting themechanical resonance point. This technique, however, is equivalent tocontrolling the supersonic motor by changing the impedance of the motor,so inherently destroys the linearity of the control system of the motor.Accordingly, stable control of a supersonic motor cannot be achieved.

At the initial stage after the first reports of supersonic motors,several proposals were made on speed control for the motors. All ofthese methods control the speed by detecting the output voltage of anamplitude sensor built in part of the supersonic motor or the absolutevalue of the amplitude or phase difference of the current flowing topiezoelectric actuators and immediately changing the resonancefrequency. These methods of control, however, do not independentlycontrol the torque, so are not suited to applications involvingformation of a servo loop as with electromagnetic motors.

A recently proposed improved method of control of a supersonic motorinvolves referring the rotational speed of the rotor against data storedas a table in a memory of a controller to calculate in reverse thetorque produced and then changing the reference speed value. This methoddoes in the end control the drive voltage or the resonance frequency,but requires detection of the rotational speed of the rotor or involvesindirect parameters derived from the computation using the data storedas a table in the memory, is susceptible to noise, and cannot really besaid to directly control the torque, so is still wanting in terms offormation of a servo loop.

In this way, the methods of control of supersonic motors proposed andcommercialized up until now have not be well suited to cooperativeoperation of a supersonic motor built in as part of a commerciallyavailable robot controller.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a method and apparatusfor control of a supersonic motor able to easily and stably control asupersonic motor.

A more specific object of the present invention is to provide a methodand apparatus for control of a supersonic motor forming a servo loop.

The inventors analyzed equivalent circuits of supersonic motors and as aresult discovered that if certain conditions were satisfied, the torqueof a supersonic motor could be expressed as a voltage.

That is, they discovered that

(a) by setting the mechanical inductance (L₀) and mechanical capacitance(C₀) so as to cancel each other out by series resonance so as to enableoperation of the supersonic motor at the mechanical resonance point ofits vibrator and

(b) by further configuring a parallel resonance tank circuit using anexternal inductance (L_(d)) so as to cancel out an electrostaticcapacitance (C_(d)),

(c) the voltage supplied becomes proportional to the torque.

Specifically, the load of the supersonic motor is always varied, but, inthe torque control circuit of the present invention, the circuitconfigurations of the control circuit for the supersonic motor and theinner equivalent circuit of the supersonic motor are perfectlysymmetric, and thus, at the both sides of the load, a voltagesubstantially equal to the reference voltage can be applied. Namely, therelationship between the circuit configuration of the control circuit ofthe supersonic motor and the circuit configuration of the innerequivalent circuit of the supersonic motor corresponds to the well known"voltage mirror circuit", and in the present invention, the voltage isequivalent to the torques, in this description, the circuit of thepresent invention is called as "torque mirror circuit" or "torquesymmetric circuit." The present invention has the advantage that thesupersonic motor can be controlled in response to the torque.

The present invention is based on this discovery.

The method for control for a supersonic motor of the present inventioncomprises controlling to zero a phase difference of a voltage suppliedto piezoelectric actuators of the supersonic motor and a current flowingto the vibrator so as to match a mechanical resonance point of avibrator of the supersonic motor.

Alternatively, the method of control for a supersonic motor of thepresent invention comprises detecting an amplitude of a vibrator of thesupersonic motor and controlling to zero the phase difference between avoltage showing the detected amplitude of the vibrator and a voltagesupplied to piezoelectric actuators of the supersonic motor.

Preferably, the voltage supplied to the piezoelectric actuators drivingthe supersonic motor is controlled in accordance with a drive torque ofthe supersonic motor.

In the above, first, the mechanical inductance (L₀) and the mechanicalcapacitance (C₀) are set so as to cancel each other out under seriesresonance conditions. This is normal practice to operate a supersonicmotor under minimum loss conditions. Next, a parallel resonance tankcircuit is configured using an external inductance (L_(d)) provided atthe outside of the supersonic motor so as to cancel out theelectrostatic capacitance (C_(d)). By this, the supersonic motor ends upwith just a mechanical arm resistance component and it is enough tocontrol the state of connection of a load to the resistance, so thecontrol becomes extremely simple. At this time, the supplied voltage isproportional to the torque, so by controlling the supplied voltagesupplied to the piezoelectric actuators, it is possible to control thetorque of the supersonic motor.

Alternatively, the method of control for a supersonic motor of thepresent invention comprises matching a mechanical resonance point of avibrator of the supersonic motor, providing a tank circuit having aninductance component in a resonant relationship with an electrostaticcapacitance of the vibrator, and controlling a voltage supplied topiezoelectric actuators of the supersonic motor in accordance with adrive torque of the supersonic motor.

Alternatively, the method of control for a supersonic motor of thepresent invention comprises matching a mechanical resonance point of avibrator of the supersonic motor, providing a tank circuit having aninductance component in a resonant relationship with an electrostaticcapacitance of the vibrator, using a circuit comprised of a mechanicalarm resistance of the supersonic motor and a torque-mirror circuit andapplying a reference voltage corresponding to a reference torque topiezoelectric actuators of the supersonic motor, and controlling thetorque of the supersonic motor.

Further, the method of control for a supersonic motor of the presentinvention may have a speed control loop added at the outside of thetorque mirror circuit for speed control of the supersonic motor.

Further, the method of control for a supersonic motor of the presentinvention may have a position control loop added at the outside of thespeed control loop added at the outside of the torque mirror circuit forposition control of the supersonic motor.

Note that instead of the method of (a) controlling to zero the phasedifference of the voltage supplied to the piezoelectric actuators of thesupersonic motor and the current flowing to the vibrator so as to matchthe mechanical resonance point of the vibrator of the supersonic motor,it is also possible to (b) provide an amplitude sensor for detecting theamplitude of the vibrator of the supersonic motor and control to zerothe phase difference between the output voltage of the amplitude sensorand the voltage supplied to the piezoelectric actuator.

According to the above methods of control, it becomes possible to form aservo loop with the supersonic motor in the same way as with anelectromagnetic motor.

Alternatively, according to the invention, there is provided anapparatus for control of a supersonic motor comprised of a vibrator, arotor arranged at one surface of the vibrator, and at least twopiezoelectric actuators provided a predetermined phase apart at theother surface of the vibrator, wherein a tank circuit having aninductance component having a resonant relationship with anelectrostatic capacitance of the supersonic motor is connected to thesupersonic motor, and the apparatus for control of a supersonic motor isprovided with a circuit comprising a mechanical arm resistance of thesupersonic motor and a torque-mirror circuit, a torque-voltageconverting means for converting a reference torque to a correspondingreference voltage, and a means for supplying the converted voltage fromthe torque-voltage converting means to the two piezoelectric actuators.

Preferably, the torque-mirror circuit is comprised of a circuit formultiplying the mechanical arm resistance and a mechanical arm currentof the supersonic motor.

Preferably, the supersonic motor is provided, before or after thetorque-voltage converting means, with an adding means for adding anoffset to the reference voltage.

Further, according to the invention, there is provided an apparatus forspeed control of a supersonic motor comprised of a vibrator, a rotorarranged at one surface of the vibrator, and at least two piezoelectricactuators provided a predetermined phase apart at the other surface ofthe vibrator, wherein a tank circuit having an inductance componenthaving a resonant relationship with an electrostatic capacitance of thesupersonic motor is connected to the supersonic motor, and the apparatusfor control of a supersonic motor is provided with a circuit comprisinga mechanical arm resistance of the supersonic motor and a torque-mirrorcircuit, a torque-voltage converting means for converting a referencetorque to a corresponding reference voltage, a means for supplying theconverted voltage from the torque-voltage converting means to the twopiezoelectric actuators, a means for detecting a speed signal of thesupersonic motor, a speed error calculating means for calculating anerror between a reference speed signal and the detected speed signal,and a speed-torque converting means for converting the speed error tothe reference torque.

Alternatively, according to the invention, there is provided anapparatus for position control of a supersonic motor supersonic motorcomprised of a vibrator, a rotor arranged at one surface of thevibrator, and at least two piezoelectric actuators provided apredetermined phase apart at the other surface of the vibrator, whereina tank circuit having an inductance component having a resonantrelationship with an electrostatic capacitance of the supersonic motoris connected to the supersonic motor, and the apparatus for control of asupersonic motor is provided with a circuit comprising a mechanical armresistance of the supersonic motor and a torque-mirror circuit, atorque-voltage converting means for converting a reference torque to acorresponding reference voltage, a means for supplying the convertedvoltage from the torque-voltage converting means to the twopiezoelectric actuators, a means for detecting a speed signal of thesupersonic motor, a speed error calculating means for calculating anerror between a reference speed signal and the detected speed signal, aspeed-torque converting means for converting the speed error to thereference torque, a means for detecting a position of the rotor of thesupersonic motor, a position error calculating means for calculating anerror between the detected position signal and a reference positionsignal, and a position-speed converting means for converting thecalculated position error signal to the reference speed signal andsupplying it to the speed error signal calculating means.

Alternatively, according to the invention, there is provided anapparatus for control of a supersonic motor comprised of a vibrator, arotor arranged at one surface of the vibrator, and at least twopiezoelectric actuators provided a predetermined phase apart at theother surface of the vibrator, provided with a means for matching amechanical resonance point of the vibrator, a tank circuit having aninductance component having a resonant relationship with anelectrostatic capacitance of the vibrator, and a means for controllingthe voltage supplied to piezoelectric actuators of the supersonic motorin accordance with a drive torque of the supersonic motor.

Alternatively according to the invention, there is provided an apparatusfor control of a supersonic motor comprised of a vibrator, a rotorarranged at one surface of the vibrator, and at least two piezoelectricactuators provided a predetermined phase apart at the other surface ofthe vibrator, provided with a means for matching a mechanical resonancepoint of the vibrator, a tank circuit having an inductance componenthaving a resonant relationship with an electrostatic capacitance of thevibrator, and a means for using a circuit comprised of a mechanical armresistance of the supersonic motor and a torque-mirror circuit andsupplying a reference voltage corresponding to the reference torque tothe piezoelectric actuators of the supersonic motor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 gives views of a supersonic motor for explaining the basicprinciple of the motor to which the present invention is applied,wherein FIG. 1(A) is a cross-sectional view of a supersonic motor 10,FIG. 1(B) is a view for explaining the principle of its basic operation,and FIG. 1(C) is a view for explaining the basic operation;

FIG. 2 is a view of an equivalent circuit of the supersonic motor shownin FIGS. 1;

FIG. 3 is a view illustrating the principle of two-phase drive of thesupersonic motor;

FIGS. 4(A) to 4(C) are views illustrating a simplified version of theequivalent circuit shown in FIG. 2;

FIG. 5 is a graph of the static characteristic of a supersonic motor inthe equivalent circuit of FIG. 4(C);

FIG. 6 is a view of a basic equivalent circuit for the driving thesupersonic motor;

FIG. 7 is a view of an equivalent circuit of a torque control circuit ofa supersonic motor using the torque-mirror circuit of the presentinvention; and

FIG. 8 is a view of an equivalent circuit of the configuration forperforming speed control and position control and representing anexpansion of the circuit of FIG. 7.

BEST MODE FOR WORKING THE INVENTION

First Embodiment

Basic Principle of Supersonic Motor

FIG. 1(A) is a schematic cross-sectional view of a supersonic motor towhich the present invention is applied. FIGS. 1(B) and (C) are views forexplaining the basic operation of the supersonic motor shown in FIG.1(A).

The supersonic motor 10 shown in FIG. 1(A) includes a vibrator 11, arotor 12 arranged at and rotating in press-contact with a surface of thevibrator 11, and two piezoelectric actuators arranged at the oppositesurface of the vibrator 11 and supplying vibration to the vibrator 11,that is, the A-phase piezoelectric actuator 13 and the B-phasepiezoelectric actuator 14.

The A-phase piezoelectric actuator 13 and the B-phase piezoelectricactuator 14 are adhered to the surface of the vibrator 11 shifted 1/2 apitch apart. These two piezoelectric actuators 13 and 14 drive thesupersonic motor. The A-phase piezoelectric actuator 13 and the B-phasepiezoelectric actuator 14 are conjugate with each other, that is,equivalent. A sine wave voltage and a cosine wave voltage are suppliedto these piezoelectric actuators. Note that the progressive waveresulting from this drive system is called a "pseudo progressive wave".

This supersonic motor 10 operates based on the following principle.

If voltages shifted exactly π/2 in phase, that is, sine wave and cosinewave voltages, are supplied to the A-phase piezoelectric actuator 13 andthe B-phase piezoelectric actuator 14 adhered to the surface of thevibrator 11 shifted 1/2 a pitch, two standing waves are produced shiftedin phase by exactly π/2 at the adhesion position of the A-phasepiezoelectric actuator 13 and the adhesion position of the B-phasepiezoelectric actuator 14. When these two standing waves are combined,the progressive wave PW expressed by the following is produced:

    A sin (ωt-kx)

Due to this progressive wave PW, the vibrator 11 undergoes repeatedelliptical motion as expressed by the later mentioned equation 4. As aresult, the rotor 12 rotates toward the direction DP. That is, the rotor12 rotates about the vibrator 11. Due to this, the supersonic motor 10functions as a motor.

An actual supersonic motor has a rotor which is modified in various waysto smooth the frictional drive so the mechanical equivalent circuitbecomes complicated.

Seen from the perspective of control, the conditions for stable drivediffer for each supersonic motor due to the difference in shape of themotor, but here an ideal supersonic motor is assumed for simplificationof the discussion.

The supersonic motor 10 shown in FIG. 1(A) is assumed to meet thefollowing conditions of an ideal supersonic motor:

(1) There is no overall slip between the vibrator 11 and the rotor 12.

That is, there is a proportional relationship between the amplitude ofthe vibrator 11 and the speed of the rotor 12.

(2) The voltage for starting the rotation under no-load conditions issubstantially constant and not that large.

That is, if the efficiency of the supersonic motor 10 becomes poor, thevibrator 11 will not rotate regardless of the vibration.

If an equivalent circuit is used for this ideal supersonic motor, theneven if there are various shapes of supersonic motors, there is theadvantage that it becomes possible to apply the model based on thisequivalent circuit for most supersonic motors and, further, to apply itto piezoelectric actuators other than supersonic motors.

The present invention in principle can be applied to a linear supersonicmotor, a complex vibrator, or a progressive wave type supersonic motor,but the following description will be made referring to a progressivewave type supersonic motor as a typical example.

FIG. 1(B) is a conceptual view of the supersonic motor 10 shown in FIG.1(A), that is, the well known progressive wave type supersonic motor.

If voltages shifted exactly π/2 in phase are applied to the A-phasepiezoelectric actuator 13 and the B-phase piezoelectric actuator 14 inthis way and a progressive wave vibration expressed by A sin (ωt-kw) isgiven to the plate-beam-like vibrator 11, elliptical vibration is causedby the progressive wave PW at the surface of the plate-beam-likevibrator 11 (the elliptical motion is shown by the later mentionedequation 4), a drive power DP is produced in a direction opposite to thepropagation direction of the progressive wave, and the rotor 12 is madeto move in a certain direction with respect to the vibrator 11. Therotor 12 is moved at the portion corresponding to the peak of thevibration produced at the vibrator 11.

The displacement u(t,x) in the flexing direction of the plate-beam-likevibrator 11 is shown by the following equation 1:

    u(t,x)=A cos (ωt+kx)                                 (1)

where, u(t,x): Position in the x-direction at the time t

A: Force factor

ω: Angular velocity (ω=2 πf)

k: Constant

Breaking down equation 1 and expressing it by two phases, the followingequation 2 is obtained:

    A cos (ωt+kx)=A cos ωt· cos kx-A sin ωt· sin kx=A cos ωt· sin (π/2-kx)-A sin ωt· sin kx                                 (2)

Equation 2 means that when sine/cosine wave vibrations of differentphases are added at 90° intervals at locations shifted spatially by 90°(1/2 pitch), the result is a progressive wave.

The first term of equation 2 is the A-phase component due to the A-phasepiezoelectric actuator 13, while the second term is the B-phasecomponent due to the B-phase piezoelectric actuator 14.

The velocity component (αu_(x) /αt) of the points of the x-axis, thatis, the propagation direction, is expressed by equation 3. ##EQU1##

Note that the elliptical motion is given by the following equation 4:##EQU2##

The A-phase piezoelectric actuator 13 and B-phase piezoelectric actuator14 have the ability to change electrical energy into mechanicaldisplacement. This is shown by the piezoelectric equations 5 and 6.These piezoelectric equations are the basic equations when configuringan equivalent circuit as seen from the standpoint of a piezoelectricmaterial.

    F=Zv-AV                                                    (5)

    I=Av÷Yd·V                                     (6)

where, F: Force (vector)

v: Velocity (vector)

Z: Mechanical impedance proportional to the velocity

A: Force factor (force-electrical conversion ability of piezoelectricactuator)

V: Voltage applied to piezoelectric actuators

I: Current flowing to the piezoelectric actuators

Yd: Admittance as a capacitor (electrostatic capacitance=Cd), whereYd=jωCd

From equations 5 and 6, it will be understood that the force Fcorresponds to the voltage V supplied to the piezoelectric actuators 13and 14, while the velocity v corresponds to the current I flowing to thepiezoelectric actuators.

FIG. 2 shows an equivalent circuit. The meaning of the symbols in FIG. 2are as follows:

C_(d) : Electrostatic capacitance as capacitor

i_(d) : Electrostatic capacitance current

i_(m) : Mechanical arm current (corresponding to velocity v)

L₀ : Mechanical inductance (mass m)

C₀ : Mechanical capacitance (constant 1/k)

r₀ : Mechanical arm resistance at no-load (frictional resistance atno-load proportional to velocity=R)

V_(P) : Voltage of battery or power outlet (corresponding to force ortorque)

Z_(L) : Load resistance connected to vibrator or rotor A load resistanceZ_(L) equal to 0 means no-load operation. The current I_(m) flowing tothe load resistance Z_(L) means the mechanical or vibrator velocity vand has a vibration component of the angular frequency ω). The angularfrequency ω is typically from 10 to 180 kHz.

In the case of a progressive wave type supersonic motor, part of thiscycle is used to take out just the force in a certain direction, so thee(jωt) component included in the supplied voltage does not appear andonly the voltage V_(P) (t) of the amplitude function acts on the loadresistance Z_(L). The A-phase and the B-phase are conjugate, so the samewaveform appears at each phase.

Note that the equivalent circuit shown in FIG. 2 has a reverserelationship of the current (torque) and voltage (velocity) of anequivalent circuit of a DC servo motor.

Here, an explanation will be made of the progressive wave and thedriving of the A-phase piezoelectric actuator and the B-phasepiezoelectric actuator.

FIG. 3 illustrates the relationship between the movement of theprogressive wave and the velocity component in the x-direction(direction of propagation) of the vibration surfaces of the A-phase andB-phase piezoelectric actuators along with the elapse of time. Thevibrator (rotor) is in contact at the portion shown by the bold line inthe figure.

As shown in the following Table 1, the voltage and current are the samein phase at the phase state in the equivalent circuit shown in FIG. 2,that is, the force produced and the velocity at the surfaces of theA-phase piezoelectric actuator 13 and B-phase piezoelectric actuator 14are the same in phase.

                  TABLE 1    ______________________________________                  A-phase   B-phase    ______________________________________    Voltage         cosωt sinωt    Current         cosωt sinωt    Z velocity      Cosωt sinωt    x velocity      cos.sup.2 ωt                                sin.sup.2 ωt    Torque          cos.sup.2 ωt                                sin.sup.2 ωt    Power           cos.sup.2 ωt                                sin.sup.2 ωt    ______________________________________

When a viscoelastic body is used for the rotor frictional member 12,elastic frictional forces proportional to the surface velocities of theA-phase piezoelectric actuator 13 and B-phase piezoelectric actuator 14are produced. These forces are combined and the rate of decrease of thevelocity is determined. From this and the integral of the velocity inthe x-direction calculated from FIG. 3, the drive voltages, currents,surface velocities, and torque distribution ratios of the piezoelectricactuators 13 and 14 at the different phases are found as shown inTable 1. From this, it is understood that the A-phase and B-phase arecompletely equivalent and when combined give a constant torque andvelocity.

Some ideas for further simplifying the equivalent circuit shown in FIG.2 will be explained with reference to FIGS. 4(A) to 4(C).

FIG. 4(A) shows the parallel connection to the supersonic motor 10 of apower source 21 for supplying voltage shifted in phase by exactly π/2 tothe A-phase piezoelectric actuator 13 and B-phase piezoelectric actuator14 of the supersonic motor 10 and an electrostatic capacitance C_(D)serving as the capacitor. FIG. 4(B) is an equivalent circuit of thesame. FIG. 4(C) is a simplified version of the equivalent circuit shownin FIG. 4(B).

If f₀ is the mechanical resonance frequency of the vibrator 11, then thefollowing equations stand under series resonance conditions: ##EQU3##

The mechanical inductance L₀ and mechanical capacitance C₀ in FIG. 2cancel each other out under series resonance conditions since theirimpedances are equal and so it may be considered that there are nomechanical inductance L₀ and mechanical capacitance C₀.

Further, the electrostatic capacitance C_(d) is the pure electrostaticcapacitance component, so by configuring the external inductance L_(d)and the tank circuit (parallel resonance circuit) 23 (FIG. 4B) to givethe following equation, the impedances of C_(d) and L_(d) become equaland are cancelled out so therefore it may be considered that there is noelectrostatic capacitance C_(d). ##EQU4##

As explained above,

(a) by setting the mechanical inductance L₀ and mechanical capacitanceC₀ so as to cancel each other out under series resonance conditions soas to enable operation at the mechanical resonance point of the vibrator11 and

(b) by further configuring a tank circuit 23 using an externalinductance L_(d) so as to cancel out an electrostatic capacitance C_(d),

the equivalent circuit of the vibrator 11 in the end becomes the simpleconfiguration shown by the mechanical arm resistance r₀ and the loadresistance Z_(L) of the load 24 as shown in FIG. 4(C).

The rotor 12 has a demodulation function, so the circuit shown in FIG.4(C), as illustrated in FIG. 5, can be used as is as the equation forthe static characteristic of the supersonic motor 10. That is, thesimplified equivalent circuit shows that the voltage V_(P) (t) from thepower source 21 is proportional to the torque. It is learned that at theabove two resonance states, it is possible to control the voltage V_(P)(t) of the power source 21 so as to control the torque of the supersonicmotor 10.

The current i_(m) flowing to the A-phase piezoelectric actuator 13 andB-phase piezoelectric actuator 14 can be measured (detected), so avoltage V₀ =V_(r) +V_(i) comprised of the supersonic motor internalvoltage V_(i) =i_(m) ×r₀, which is a value obtained by detecting thecurrent i_(m) in advance from the admittance circle and multiplying itwith the mechanical arm resistance r₀, plus the reference voltage V_(r)is supplied to the two ends of the A-phase piezoelectric actuator 13 andB-phase piezoelectric actuator 14.

The circuit configuration realizing this is shown in FIG. 6.

This supersonic motor control circuit 100 includes a multiplier(circuit) 101 for multiplying the coefficient K_(TV) with the torquecommand to convert it to a reference voltage V_(r) according to thetorque command T_(R), an adder (circuit) 103 for adding to the referencevoltage a no-load stopping voltage ΔV₀ as an offset to produce amodified reference voltage V_(r) ', an adder (circuit) 105 for adding tothe modified reference voltage V_(r) ' a supersonic motor internalvoltage V_(i) =i_(m) ×r₀, a current detection circuit 109 for detectingthe current flowing to the A-phase piezoelectric actuator 13 and B-phasepiezoelectric actuator 14 of the supersonic motor 10, and a multiplier(circuit) 107 for multiplying the mechanical arm resistance r₀ with thecurrent i_(m) detected by the current detection circuit 109 to calculatethe supersonic motor internal voltage V_(i) =i_(m) ×r₀.

The supersonic motor 10 has connected to it a tank circuit 23.

In this example, the voltage V₀ supplied to the A-phase piezoelectricactuator 13 and B-phase piezoelectric actuator 14 of the supersonicmotor 10 is expressed by the following:

    V.sub.0 =V.sub.r '+V.sub.i =V.sub.r +ΔV.sub.0 +V.sub.I =(T.sub.R ×K.sub.TV)+ΔV.sub.0 +V.sub.I

Note that the voltage ΔV₀ for correcting the stopping voltage underengine no-load conditions (no-load stopping voltage correction voltage)supplied as the offset voltage is an option and need not be applied. Atthis time, the adder 103 becomes unnecessary and the voltage supplied tothe A-phase piezoelectric actuator 13 and B-phase piezoelectric actuator14 of the supersonic motor 10 at this time becomes as shown by thefollowing equation:

    V.sub.0 =V.sub.r +V.sub.I =(T.sub.R ×K.sub.TV)+V.sub.I

If the above voltage V₀ is supplied to the two ends of the piezoelectricactuators 13 and 14, then a torque proportional to the reference voltageV_(r) or the modified reference voltage V_(r) ' can be taken out fromthe output shaft of the supersonic motor 10.

Second Embodiment

FIG. 7 shows an equivalent circuit of the supersonic motor controlcircuit 200 illustrating the equivalent circuit 110 and mechanicalsystem 130 in the supersonic motor in addition to the configuration ofthe supersonic motor control circuit 100 of FIG. 6.

The supersonic motor control circuit 200 is comprised of the supersonicmotor control circuit 100 shown in FIG. 6, the supersonic motor internalequivalent circuit 110, and the mechanical system 130.

The supersonic motor control circuit 100, like illustrated in FIG. 6, iscomprised of a multiplier (circuit) 101 for converting to a referencevoltage V_(r) according to the torque command T_(R), a first adder(circuit) 103 for adding to the reference voltage a no-load stoppingvoltage ΔV₀ as an offset to produce a modified reference voltage V_(r)', an adder circuit 105) for adding to the modified reference voltageV_(r) ' a supersonic motor internal voltage V_(i) =i_(m) ×r₀, and amultiplier (circuit) 107 for multiplying the mechanical arm resistancer₀ with the current i_(m) to calculate the supersonic motor internalvoltage V_(i) =i_(m) ×r₀.

The equivalent circuit 110 of the inside of the supersonic motor iscomprised of an adder (circuit) 111 for adding to the stopping voltagecorrecting voltage V₀ supplied to the A-phase piezoelectric actuator 13and B-phase piezoelectric actuator 14 of the supersonic motor, outputfrom the adder (circuit) 105 a supersonic motor internal voltage V_(i)from the multiplier (circuit) 115, a voltage-torque converter 113 forconverting the added voltage V₀ '=V₀ +V_(i) from the adder (circuit) 111to a torque, a torque-current converter 117 for converting the torque Tproduced by the voltage-torque converter 114 into a current, an adder(circuit) 119 for calculating the current i_(m), and a multiplier(circuit) 115 for multiplying the mechanical arm resistance r₀ with themechanical arm current i_(m) to calculate the supersonic motor internalvoltage.

The mechanical system 130 is shown by the inertia equivalent portion 131having the inertia 1/Js and an integration system 133. The output of theinertia equivalent portion 131 is the speed (dot Θ), while the output ofthe integration system 133 is the position Θ.

The supersonic motor internal equivalent circuit 110 further has aspeed-current converter (circuit) 121 for converting the output of theinertia equivalent portion 131, that is, the speed (dot Θ), intocurrent. The output of the integration system 133 is the position Θ.

In the supersonic motor internal equivalent circuit 110, the vibrator 11of the supersonic motor shown by the voltage-torque converter 113vibrates in accordance with the voltage V₀ supplied to the A-phasepiezoelectric actuator 13 and B-phase piezoelectric actuator 14 shown inFIG. 1(A) and the rotor 12 then rotates (moves). The torque T producedin the voltage-torque converter 113 is converted to the correspondingcurrent at the torque-current converter 117. The speed-current converter(circuit) 121 converts the speed, that is, the output of the inertiaequivalent portion 131, to the corresponding current The output of thetorque-current converter 117 and the output of the speed-currentconverter (circuit 121) are added by the adder (circuit) 119 and theresult is supplied as the detected current i_(m) to the multiplier(circuit) 107 and multiplier (circuit) 115.

The load of the motor constantly changes, but in the torque control unit200, the circuit in the supersonic motor control circuit 100 comprisedby the multiplier (circuit) 107 and adder (circuit) 105 and the circuitin the supersonic motor internal equivalent circuit 110 comprised by themultiplier (circuit) 115 and the adder (circuit) 111 are completelysymmetrical, so a voltage equal to the modified reference voltage V_(R)' is supplied to the two ends of the load. This circuit 140 correspondsto a voltage mirror circuit. Since the voltage is equal to the torque,it is called a "torque-mirror circuit".

The supersonic motor control circuit 200 enables the supersonic motor,shown by the voltage-torque converter 113, to operate by the referencevoltage V_(R) modified in accordance with the reference torque T_(R).

In this way, according to the present invention, it is possible tocontrol the supersonic motor in accordance with the torque.

Third Embodiment

In the above, the explanation was made of torque control as the methodof control of a supersonic motor of the present invention, but thepresent invention may also be used for speed control or positioncontrol.

FIG. 8 is a block diagram of a control apparatus 300 for performingtorque control, speed control, and position control for a supersonicmotor.

In FIG. 8, the control apparatus 300 is provided with a torque controlloop 150 including the supersonic motor control circuit 100 shown inFIG. 7 and a torque-mirror circuit 140 shown as the supersonic motorinternal equivalent circuit 110 and a speed control loop 320 provided atthe outside of the torque control loop 150 and the inertia equivalentportion 131 of the mechanical system 130. Further, a position controlloop 310 is provided at the outside of that.

The supersonic motor control circuit 100, not shown, as shown in FIG. 7includes a multiplier (circuit) 101, an adder (circuit) 103, an adder(circuit) 105, and a multiplier (circuit) 107.

In the same way, the supersonic motor internal equivalent circuit 110,not shown, as shown in FIG. 7, includes an adder (circuit) 111, avoltage-torque converter 113, a multiplier (circuit) 115, atorque-current converter 117, an adder (circuit) 119, and aspeed-current converter (circuit) 121.

The mechanical system 130 is comprised of an inertia equivalent portion131 and an integration system 133.

The position control loop 310 includes an adder (or subtractor) 311 forcalculating the deviation (error) ΔΘ between the reference positionsignal Θ_(R) and the output position signal Θ of the integration system133 and a multiplier (position-speed converter) 313 for converting theposition error signalΔΘ to a reference speed signal (dot Θ).

The speed control loop 320 includes an adder (or subtractor) 321 forcalculating the error (deviation) between the reference speed signal(dot Θ) from the multiplier (position-speed converter) 313 and the speedsignal (dot Θ) from the inertia equivalent portion 131 and aspeed-torque conversion circuit (multiplier) 323 for converting thespeed error signal calculated at the adder (subtractor) 321 into areference torque T_(R).

For the speed signal, use is made of the integral of the mechanical armcurrent i_(m) or the output of an encoder (not shown) detecting therotational position of the supersonic motor 10. The position signal usedis either the integral of the mechanical arm current i_(m) or the outputof the encoder.

Next, the operation of the control apparatus 300 shown in FIG. 8 will beexplained.

In the control apparatus 300, the torque control loop 150 operates as aminor control loop, the speed control loop 320 at the outside controlsthe speed, and the position control loop 310 controls the position.

The torque control loop 150 controls the torque of the supersonic motor10 in accordance with the rotational operation of the torque-mirrorcircuit 140 when a reference torque T_(R) is input.

The speed control loop 320 produces a reference torque T_(R) andcontrols the speed of the supersonic motor 10 to follow that referencetorque T_(R) so that the actual speed signal (dot Θ) of the inertiaequivalent portion 131 matches the reference speed signal (dot Θ) fromthe multiplier (position-speed converter) 313.

The position control loop 310 controls the position of the supersonicmotor 10 so that the actual position signal Θ of the integration system133 matches the reference position signal Θ_(R).

The block diagram of control of FIG. 8 is equivalent to an ordinaryservo loop in an electromagnetic motor etc. Accordingly, it is possibleto form a servo loop the same as with an electromagnetic motor for asupersonic motor using the method of control of the present invention.

In particular, the frequency characteristic of the supersonic motor ofthe present invention extends up to the extremely high region.Accordingly, a supersonic motor using this method of control can beeasily incorporated into the position control loop of an FA robotmanipulator. In other words, a supersonic motor using the method ofcontrol of the present invention can be used in the same way as anelectromagnetic motor. In this case, it is possible to take advantage ofthe torque characteristic, extremely low speed rotation, and otherfeatures of a supersonic motor.

In this way, according to the present invention, it becomes possible toform a servo loop for the supersonic motor in the same way as anelectromagnetic motor. Accordingly, for example, a supersonic motor maybe used for a FA robot.

Note that in the control apparatus 300 shown in FIG. 8, speed controlalone is also possible by a configuration not including the positioncontrol loop 310. In this case, the adder shown (or subtractor) 311 andmultiplier (phase-speed converter) 313 shown in FIG. 8 may be deleted.

The present invention is not limited to the above embodiments.

For example, in the above embodiments, the explanation was made of themethod for controlling to zero the phase difference between the voltagesupplied to the supersonic motor and the current flowing to the vibratorso as to match the mechanical resonance point of the vibrator of thesupersonic motor, but as a modification of this, it is also possible toprovide an amplitude sensor for detecting the amplitude of the vibratorof the supersonic motor and control to zero the phase difference betweenoutput voltage of the amplitude sensor and the voltage supplied to thepiezoelectric actuators.

Note that an amplitude sensor is normally formed inside thepiezoelectric actuator. The volate when the charge occurring across thesensor flows through the resistance element is amplified and used as theoutput voltage.

Further, in the above embodiments, the explanation was made of aprogressive wave type supersonic motor as a representative example, butthe present invention is not limited to a progressive wave typesupersonic motor when worked and may be applied to another type ofsupersonic motor, for example, a composite supersonic motor.

According to the method of control of a supersonic motor based on thetorque-mirror system of the present invention, it is possible to controlthe torque based on the voltage supplied to the supersonic motor. Inother words, it is possible to form a servo loop similar to that for anelectromagnetic motor for the supersonic motor.

Further, according to the method of control of a supersonic motor of thepresent invention, not only is it possible to form a servo loop, but itis also possible to use the characteristics of a supersonic motor, forexample, its high torque characteristic, not able to be realized byelectromagnetic motors, for various applications.

INDUSTRIAL APPLICABILITY

The supersonic motor of the present invention can be used for variouscontrol apparatuses such as control apparatuses incorporating servoloops.

I claim:
 1. An apparatus for control of a supersonic motor containing avibrator, a rotor arranged at a first surface of the vibrator, and atleast two piezoelectric actuators provided a predetermined phase apartat a second surface of the vibrator, the apparatus comprising:means formatching a drive voltage supplied to said piezoelectric actuators with amechanical resonance point of said vibrator; a tank circuit, connectedto said supersonic motor, including an inductance component, saidinductance component having a resonant relationship with anelectrostatic capacitance of said vibrator, and a circuit including amechanical arm resistance circuit of said supersonic motor and atorque-mirror circuit, said circuit supplying a reference voltage,corresponding to a reference torque, to the piezoelectric actuators ofsaid supersonic motor.
 2. A method for controlling a supersonic motorcomprising:detecting a varying voltage amplitude corresponding to avarying vibration amplitude of a vibrator of said supersonic motor; andcontrolling to zero a phase difference between said varying voltageamplitude and a drive voltage supplied to piezoelectic actuators whichdrive said vibrator.
 3. A method of control for a supersonic motor asset forth in claim 2, wherein said drive voltage supplied to thepiezoelectric actuators is controlled in accordance with a signalindicating a desired torque of said supersonic motor.
 4. A method ofcontrolling a supersonic motor, the method comprising:matching a drivevoltage supplied to piezoelectric actuators of said supersonic motorwith a mechanical resonance point of a vibrator in said supersonicmotor; coupling a tank circuit having an inductance component in aresonant relationship with an electrostatic capacitance of saidvibrator; and controlling said drive voltage supplied to piezoelectricactuators in accordance with a signal indicating a desired torque ofsaid supersonic motor.
 5. A method of control for a supersonic motorhaving a vibrator and piezoelectric actuators, the methodcomprising:supplying a drive voltage, which is derived from a referencevoltage corresponding to a reference torque, to the piezoelectricactuators of said supersonic motor by using a circuit comprising amechanical arm resistance circuit in the supersonic motor and atorque-mirror circuit; providing a tank circuit having an inductancecomponent in a resonant relationship with an electrostatic capacitanceof said vibrator; controlling the torque of the supersonic motor inaccordance with said reference voltage by matching the drive voltagesupplied to the piezoelectric actuators in the supersonic motor with amechanical resonance point of the vibrator of said supersonic motor. 6.A method of control for a supersonic motor as set forth in claim 5,wherein a speed control loop is added at the outside of the torquemirror circuit for speed control of the supersonic motor.
 7. A method ofcontrol for a supersonic motor as set forth in claim 6, wherein aposition control loop is added at the outside of the speed control loopadded at the outside of the torque mirror circuit for position controlof the supersonic motor.
 8. An apparatus for controlling the torque of asupersonic motor, said supersonic motor being comprised of a vibrator, arotor arranged at a first surface of the vibrator, and at least twopiezoelectric actuators provided a predetermined phase apart at a secondsurface of the vibrator, said apparatus comprising:a tank circuitcoupled with the supersonic motor, said tank circuit including aninductance component having a resonant relationship with anelectrostatic capacitance of said supersonic motor; a torque voltageconverting means for converting a reference torque signal to acorresponding reference voltage; and a first circuit comprising amechanical arm resistance of said supersonic motor and a torque-mirrorcircuit for generating a control voltage corresponding to said referencevoltage from said torque-voltage converting means and directing saidcontrol voltage to said two piezoelectric actuators.
 9. An apparatus forcontrol of a supersonic motor as set forth in claim 8, wherein saidtorque-mirror circuit is comprised of a circuit multiplying saidmechanical arm resistance and a mechanical arm current of saidsupersonic motor.
 10. An apparatus for control of a supersonic motor asset forth in claim 8 or 9, wherein said supersonic motor is provided,before or after said torque-voltage converting means, with an addingmeans for adding an offset to said reference voltage.
 11. An apparatusfor control of a supersonic motor as set forth in claim 8, furthercomprising:a means for generating a speed signal indicative of a speedof said supersonic motor, a speed error calculating means forcalculating an error between a reference speed signal and said generatedspeed signal, and a speed-torque converting means for generating saidreference torque signal in accordance with said calculated speed error.12. An apparatus for control of a supersonic motor as set forth in claim11, wherein said torque-mirror circuit is comprised of a circuitmultiplying said mechanical arm resistance and a mechanical arm currentof said supersonic motor.
 13. An apparatus for control of a supersonicmotor supersonic motor as set forth in claim 11, further comprising:ameans for generating a position signal indicative of a position of therotor of said supersonic motor, a position error calculating means forcalculating an error between said generated position signal and areference position signal, and a position-speed converting means forgenerating a reference speed signal in accordance with said calculatedposition error.
 14. An apparatus for control of a supersonic motor asset forth in claim 13, wherein said torque-mirror circuit is comprisedof a circuit multiplying said mechanical arm resistance and a mechanicalarm current of said supersonic motor.
 15. An apparatus for control of asupersonic motor comprised of a vibrator, a rotor arranged at a firstsurface of the vibrator, and at least two piezoelectric actuatorsprovided a predetermined phase apart at a second of the vibrator, saidapparatus comprising:a means for matching a drive voltage supplied tosaid piezoelectric actuators with a mechanical resonance point of saidvibrator, a tank circuit, connected to said supersonic motor, having aninductance component, said inductance component having a resonantrelationship with an electrostatic capacitance of said vibrator, and ameans for controlling said drive voltage supplied to said piezoelectricactuators of said supersonic motor in accordance with a signalindicating a desired torque of said supersonic motor.